1. Field of the Invention
The present invention relates to radio receivers, particularly digital radio receivers.
2. State of the Art
Modulation can be defined as the alteration of some characteristic of a known signal or waveform, i.e., a carrier, as a function of some unknown signal or waveform that conveys information. In radio-frequency (RF) communication systems, the carrier is typically a sinusoid, and there are several methods of modulating the carrier. These include linear modulation, angle modulation, and various types of pulse modulation. Given a sinusoidal carrier described by the equation A(t)cos(.omega..sub.c t+.phi.(t)), there are two parameters, the amplitude and the phase angle, that can be varied in accordance with an information signal. Linear modulation results when the amplitude is varied as a linear function of the information signal. Angle modulation includes phase modulation and frequency modulation. If a term is included in the argument of the sinusoidal function that varies in proportion to the information signal, the result is phase modulation. If the argument is such that the difference in the instantaneous frequency and the carrier frequency is proportional to the information signal, the result is frequency modulation.
Demodulation of RF signals has typically involved a quadrature detector having two branches, an I ("in-phase") branch and a Q ("quadrature" or 90.degree. phase-shifted) branch. In the I branch, a received signal is multiplied by the cosine form of the carrier signal and then passed through a low-pass filter. In the Q branch, the received signal is multiplied by the sine form of the carrier signal and pass through a low-pass filter. Quadrature detectors of this type are linear, well-understood, and almost universally used. To obtain the information signal from the I and Q components produced by the respective I and Q branches of the quadrature detector, signal processing is performed. In particular, the phase of the signal may be obtained by taking the inverse tangent of the ratio of Q to I. The amplitude of the signal may be obtained according to the Pythagorean theorem by taking the square root of the sum of the squares of I and Q. These mathematical operations are non-linear.
Two salient observations may therefore be made concerning quadrature detection. First, detection proceeds in two steps, a first mixing step (to obtain I and Q) that is linear and a second signal processing step to which non-linearities are relegated. Second, a coordinate system conversion is first performed and then reversed. That is, the received signal, which may be readily described in polar coordinates in terms of the desired quantities of amplitude and phase, is first converted to rectangular coordinates by projecting the instantaneous signal vector in polar coordinates onto the X (I) and Y (Q) axes, and is then converted back to polar coordinates to obtain amplitude and phase. Such conversions require circuitry that occupies space and consumes power-both of which may be precious commodities, especially in mobile applications such as cellular telephones, pagers, etc. Such conversions may also entail substantial inaccuracies.
What is needed, then, is demodulation techniques that allow for space savings, power savings or increased accuracy to be obtained.